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Took a profile of the "Smart Cover" page on Apple's site using Pingdom. It's requesting 241 objects for a total page weight of 3472.6 KB! An absolute mammoth.

How does this possibly translate into a 4-second download time? I'm not even on a great connection and it still loads in seconds using my clogged-up DSL. That interactive graphic towards the middle loads over 90 images per selected cover, at around 40k each. Still, it's done at blazing speeds.

How do they do it?

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They have access to an Akamai CDN network. As well, they've optimized the site to download images as they're viewed, so until you request them, you're not using bandwidth. If you deliberately slow down your connection, you'll notice a loading spinner while that image loads. If you leave the page long enough, it's > 30MB. This is all accomplished with deferred loading

The CDN is probably the biggest thing though. High throughput, low latency will always make things load REALLY quickly

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+1; they are one of the biggest clients of Akamai, and have been for many years. long before the appStore, even before iTunes. – Javier Mar 24 '11 at 22:22
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BTW, lots of people these days call 'CDN' to just any number of static web servers; but Akamai are one of the very best, with lots of tricks to serve content as close as possible to the client. They have servers right in the datacenters of most ISPs in the world. – Javier Mar 24 '11 at 22:25
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Very interesting. I suspected the CDN, but my last employer used Akamai as well on a huge site with results that weren't nearly as amazing. Any more info on the "downloading images as they're viewed" part? Is this just a piece of JS that detects whether an image is in your viewport yet? – Jon Mar 24 '11 at 22:37
yep. it's precisely that simple. You can find a few scripts out there that'll handle that if you don't want to write it yourself – Jeremy Massel Mar 24 '11 at 23:23
Given the fact that they leave so much in terms of performance optimization on the table (see: page-speed/yslow results - gtmetrix.com/reports/www.apple.com/sYrN2NoW and compression savings - goo.gl/adOFs) it really is amazing! – Mike Hudson Mar 29 '11 at 10:14
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